Sun Zhengcai, a Politburo member, rose to the Chongqing party leadership post after the scandal-plagued exit of Bo Xilai in 2012

BEIJING—China’s Communist Party placed under investigation a senior official once seen as a possible successor to President Xi Jinping, people familiar with the matter said, in a change that allowed the Chinese leader to promote an ally.

A hastily arranged meeting of party officials on Saturday in the inland city of Chongqing announced that Sun...